Illumination system of a microlithographic projection exposure apparatus

ABSTRACT

An illumination system of a microlithographic projection exposure apparatus includes an optical raster element configured to produce a plurality of secondary light sources located in a system pupil surface. The optical raster element has a plurality of light entrance facets, each being associated with one of the secondary light sources. A beam deflecting device includes a beam deflection array of reflective or transparent beam deflecting elements, each being configured to illuminate a spot on one of the light entrance facets at a position that is variable by changing a deflection angle produced by the beam deflecting element. A control unit is configured to control the beam deflection elements such that variable light patterns assembled from the spots can be formed on at least one of the plurality of light entrance facets.

CROSS-RELATED APPLICATIONS

This application is a continuation of, and claims benefit under 35 USC120 to, international application PCT/EP2009/004574, filed Jun. 25,2009, which claims benefit of European Application No. 08012815.0, filedJul. 16, 2008 and U.S. Ser. No. 61/081,163, filed Jul. 16, 2008.International application PCT/EP2009/004574 is hereby incorporated byreference in its entirety.

FIELD

The disclosure generally relates to illumination systems forilluminating a mask in microlithographic exposure apparatus, and inparticular to such systems including an array of mirrors or other beamdeflecting elements. The disclosure also relates to a method ofoperating such systems.

BACKGROUND

Microlithography (also called photolithography or simply lithography) isa technology for the fabrication of integrated circuits, liquid crystaldisplays and other microstructured devices. The process ofmicrolithography, in conjunction with the process of etching, is used topattern features in thin film stacks that have been formed on asubstrate, for example a silicon wafer. At each layer of thefabrication, the wafer is first coated with a photoresist which is amaterial that is sensitive to radiation, such as deep ultraviolet (DUV)light. Next, the wafer with the photoresist on top is exposed toprojection light in a projection exposure apparatus. The apparatusprojects a mask containing a pattern onto the photoresist so that thelatter is only exposed at certain locations which are determined by themask pattern. After the exposure the photoresist is developed to producean image corresponding to the mask pattern. Then an etch processtransfers the pattern into the thin film stacks on the wafer. Finally,the photoresist is removed. Repetition of this process with differentmasks results in a multi-layered microstructured component.

A projection exposure apparatus typically includes an illuminationsystem for illuminating the mask, a mask stage for aligning the mask, aprojection objective, and a wafer alignment stage for aligning the wafercoated with the photoresist. The illumination system illuminates a fieldon the mask that may have the shape of a rectangular or curved slit, forexample.

In current projection exposure apparatus a distinction can be madebetween two different types of apparatus. In one type each targetportion on the wafer is irradiated by exposing the entire mask patternonto the target portion in one go. Such an apparatus is commonlyreferred to as a wafer stepper. In the other type of apparatus, which iscommonly referred to as a step-and-scan apparatus or scanner, eachtarget portion is irradiated by progressively scanning the mask patternunder the projection beam along a scan direction while synchronouslymoving the substrate parallel or anti-parallel to this direction. Theratio of the velocity of the wafer and the velocity of the mask is equalto the magnification of the projection objective, which is usuallysmaller than 1, for example 1:4.

It is to be understood that the term “mask” (or reticle) is to beinterpreted broadly as a patterning mechanism. Commonly used maskscontain transmissive or reflective patterns and may be of the binary,alternating phase-shift, attenuated phase-shift or various hybrid masktype, for example. However, there are also active masks, e.g. masksrealized as a programmable mirror array. Also programmable LCD arraysmay be used as active masks.

As the technology for manufacturing microstructured devices advances,there are ever increasing demands also on the illumination system.Ideally, the illumination system illuminates each point of theilluminated field on the mask with projection light having a welldefined irradiance and angular distribution. The term angulardistribution describes how the total light energy of a light bundle,which converges towards a particular point in the mask plane, isdistributed among the various directions of the rays that constitute thelight bundle.

The angular distribution of the projection light impinging on the maskis usually adapted to the kind of pattern to be projected onto thephotoresist. For example, relatively large sized features may involve adifferent angular distribution than small sized features. The mostcommonly used angular distributions of projection light are referred toas conventional, annular, dipole and quadrupole illumination settings.These terms refer to the irradiance distribution in a system pupilsurface of the illumination system. With an annular illuminationsetting, for example, only an annular region is illuminated in thesystem pupil surface. Thus there is only a small range of angles presentin the angular distribution of the projection light, and thus all lightrays impinge obliquely with similar angles onto the mask.

Different approaches are known in the art to modify the angulardistribution of the projection light in the mask plane so as to achievethe desired illumination setting. In the simplest case a stop(diaphragm) including one or more apertures is positioned in a pupilsurface of the illumination system. Since locations in a pupil surfacetranslate into angles in a Fourier related field plane such as the maskplane, the size, shape and location of the aperture(s) in the systempupil surface determines the angular distributions in the mask plane.However, any change of the illumination setting involves a replacementof the stop. This makes it difficult to finally adjust the illuminationsetting, because this would involve a very large number of stops thathave aperture(s) with slightly different sizes, shapes or locations.

Many common illumination systems therefore include adjustable elementsthat make it possible, at least to a certain extent, to continuouslyvary the illumination of the pupil surface. Conventionally, a zoomaxicon system including a zoom objective and a pair of axicon elementsare used for this purpose. An axicon element is a refractive lens thathas a conical surface on one side and is usually plane on the oppositeside. By providing a pair of such elements, one having a convex conicalsurface and the other a complementary concave conical surface, it ispossible to radially shift light energy. The shift is a function of thedistance between the axicon elements. The zoom objective makes itpossible to alter the size of the illuminated area in the pupil surface.

For further increasing the flexibility in producing different angulardistribution in the mask plane, it has been proposed to use mirrorarrays that illuminate the pupil surface. In EP 1 262 836 A1 the mirrorarray is realized as a micro-electromechanical system (MEMS) includingmore than 1000 microscopic mirrors. Each of the mirrors can be tilted intwo different planes perpendicular to each other. Thus radiationincident on such a mirror device can be reflected into (substantially)any desired direction of a hemisphere. A condenser lens arranged betweenthe mirror array and the pupil surface translates the reflection anglesproduced by the mirrors into locations in the pupil surface. This knownillumination system makes it possible to illuminate the pupil surfacewith a plurality of circular spots, wherein each spot is associated withone particular microscopic mirror and is freely movable across the pupilsurface by tilting this mirror.

Illumination systems are also known from, for example, US 2006/0087634A1, U.S. Pat. No. 7,061,582 B2 and WO 2005/026843 A2.

The geometry of the field illuminated in the mask plane is usuallydetermined by a plurality of components. One of the most importantcomponents in this respect is an optical raster element which produces aplurality of secondary light sources in the system pupil plane. Theangular distribution of the light bundles emitted by the secondary lightsources is directly related to the geometry of the field illuminated inthe mask plane. By suitably determining the optical properties of theoptical raster element, for example the refractive power in orthogonaldirections, it is possible to obtain the desired field geometry.

Usually it is desired that the geometry of the illuminated field can bevaried at least to a certain extent. Since the optical properties of theoptical raster element cannot be changed easily, a field stop isprovided that is imaged by a field stop objective on the mask. The fieldstop usually includes a plurality of blades that can be individuallymoved so as to delimit the field illuminated in the mask. The field stopalso ensures sharp edges of the illuminated field. In apparatus of thescanner type an adjustable field stop is used to open and shut theilluminated field at the beginning and the end of each scan process,respectively.

If the geometry of the illuminated field is varied with the help of anadjustable field stop, light losses are inevitable because a portion ofthe projection light is blocked by the blades of the field stop.

SUMMARY

The disclosure provides an illumination system which makes it possibleto vary the geometry of the illuminated field with smaller light losses.

In some embodiments, an illumination system includes a primary lightsource, a system pupil surface, and a mask plane in which a mask to beilluminated can be arranged. The system further includes an opticalraster element which is configured to produce a plurality of secondarylight sources located in the system pupil surface. The optical rasterelement has a plurality of light entrance facets each being associatedwith one of the secondary light sources. A beam deflecting device of theillumination system includes a beam deflection array of reflective ortransparent beam deflecting elements. Each beam deflecting element isconfigured to illuminate a spot on one of the light entrance facets at aposition that is variable by changing a deflection angle produced by thebeam deflection element. A control unit is configured to control thebeam deflection elements such that variable light patterns assembledfrom the spots can be formed on at least one of the plurality of lightentrance facets.

The disclosure exploits the fact that locations on the light facets ofthe optical raster element translate into angles of the light emitted bythe secondary light sources. Thus each light pattern illuminated on afacet is associated with a different angular distribution of lightemitted by the secondary light source associated with the light entrancefacet. Since the angular distribution of a secondary light sourcetranslates back into a geometry of the illuminated field in the maskplane, the light patterns illuminated on the light entrance facets havea one to one correspondence to the geometry of the illuminated field inthe mask plane. In the absence of optical aberrations, the illuminatedfield is a superposition of images of the light patterns formed on thelight entrance facets of the optical raster element.

The provision of the beam deflecting device makes it possible toaccurately vary the locations of the spots which are illuminated on thelight entrance facets of the optical raster element. In order to producedifferent light patterns, the spots illuminated by the beam deflectingelements have a total area that is sufficiently smaller than a maximumtotal area of a light entrance facet. Preferably the spot area is atleast 5 times, more preferably at least 10 times, and most preferably atleast 20 times, smaller than the maximum total area of any of the lightentrance facets.

In contrast to certain known illumination systems including beamdeflection devices, in which the spots are determined such that they atleast substantially illuminate the complete area of the light entrancefacets, the substantially smaller spot size of the present disclosure(if compared to the area of the facets) makes it possible to produce akind of illuminated fine structure on the optical raster element inwhich the geometry of the illuminated field is encoded.

By changing this fine structure, the geometry of the illuminated fieldmay be varied without incurring substantial light losses, as would bethe case if an adjustable field stop is used to this end. It is evenpossible to completely dispense with the field stop and also with thefield stop objective, which considerably simplifies the overall layoutof the illumination system. If a field stop is nevertheless provided,the varying geometry will be mainly determined by the beam deflectingdevice, whereas the field stop only ensures sharp edges, but blocks onlya very small portion of the projection light.

The spots illuminated by the beam deflecting elements on the lightentrance facets may have any arbitrary geometry. These geometries do nothave to be identical for all spots.

For example, rectangular spots with a different spot size, or a mixtureof rectangular and triangular spots may be envisaged. Preferably thespots have geometries that can be assembled to larger areas such that noor very small gaps remain between adjacent spots. Usually the geometryof the spots illuminated on the light entrance facets depends mainly onthe angular distribution of the light impinging on the micro-mirrors. Anarray of microlenses arranged in front of the micro-mirrors may be usedto produce an angular distribution that results in the desired spotgeometry.

In one embodiment the spots have an at least substantially rectangulargeometry. This is advantageous because such spots can be lined up alonglines which may then (subsequently) combine to larger rectangular areasilluminated on a single light entrance facet.

Usually it will be preferred if, at a given instant, the light patternsproduced on all illuminated light entrance facets are identical. Thisensures that all secondary light sources illuminate the same field inthe mask plane so that the intensity is at least substantially identicalthroughout the illuminated field. However, in other cases it may bedesired to have a certain intensity profile within the illuminatedfield. For example, in some projection exposure apparatus of the scannertype it is desired to have an intensity profile with a smoothlyincreasing and decreasing intensity at the edges extending perpendicularto a scan direction of the apparatus. In this case there should bedifferent light patterns on the illuminated light entrance facets at agiven instant.

In one embodiment the control unit is configured to control the beamdeflection elements such that the lengths of the light patterns alongthe scan direction are gradually varied during a scan process of theapparatus, whereas the length of the light patterns along a directionperpendicular to the scan direction remains constant. This may be usedto imitate the function of an adjustable field stop at the beginning andthe end of each scan process.

If a pulsed laser is used as a primary light source in the illuminationsystem, the variation of the light patterns should be synchronized withthe pulse rate of the laser.

Such a synchronization may not be necessary if at least some of thelight entrance facets are provided with blinds. This makes it possibleto continuously move the light entrance patterns over the light entrancefacets so that the blinds will gradually obstruct more and more of thespots. This, in turn, results in a gradual reduction of the size of thelight patterns and thus of the field illuminated in the mask plane.

In this context it may be envisaged that at least one light entrancefacet is provided with a pair of blinds that are arranged on oppositesides of the light entrance facet. Such a configuration is particularlyuseful if the illuminated field has to increase and to decrease alongthe scan direction at the beginning and the end, respectively, of thescan process.

According to some embodiments, the illumination system includes adiaphragm which is arranged in close proximity to the beam deflectiondevice. An actuator is provided for moving the diaphragm parallel to thescan direction of a microlithographic projection exposure apparatus. Ifthe diaphragm moves continuously or intermittently into the lightbundles associated with the beam deflection elements, an increasing ordecreasing portion of these light bundles will be blocked by thediaphragm. If, for example, first those light bundles are blocked thatcontribute exclusively to the illumination of a certain area in thefield illuminated on the mask, this area will become dark if thediaphragm starts moving. In one embodiment this area is a line extendingperpendicular to the scan direction so that the movement of thediaphragm results in the typical field geometry variation at thebeginning and the end of each scan process.

The beam deflection elements may be configured as mirrors which can betilted by two tilt axes forming an angle therebetween. In anotherembodiment, the beam deflection elements are electro-optical oracousto-optical elements.

In some embodiments, the disclosure provides a method that includes:

-   a) providing an illumination system of a microlithographic    projection exposure apparatus, wherein the illumination system    includes an optical raster element having a plurality of light    entrance facets;-   b) producing light patterns, which are assembled from individual    spots, on the light entrance facets of the optical raster element;-   c) determining that the geometry of a field to be illuminated in a    mask plane shall change;-   d) varying the light patterns on the light entrance facets by    rearranging and/or removal and/or adding spots.

The remarks made above relating to the illumination system according tothe present disclosure also apply as appropriate to the methods.

BRIEF DESCRIPTION OF THE DRAWINGS

Various features and advantages of the present disclosure may be morereadily understood with reference to the following detailed descriptiontaken in conjunction with the accompanying drawing in which:

FIG. 1 is a perspective and considerably simplified view of a projectionexposure apparatus in accordance with the present disclosure;

FIG. 2 is a meridional section through an illumination system containedin the projection exposure apparatus shown in FIG. 1;

FIG. 3 is a perspective view of a mirror array contained in theillumination system of FIG. 2;

FIG. 4 is an enlarged cut-out of FIG. 2 showing the mirror array of FIG.3 and first microlenses of an optical raster element;

FIG. 5 a is an enlarged cut-out of FIG. 2 showing first and secondmicrolenses of the optical raster element and a condenser lens;

FIG. 5 b is a top view on a system pupil surface in which secondarylight sources are formed;

FIG. 6 is a top view on the optical raster element with some lightentrance facets illuminated by rectangular light patterns;

FIG. 7 is a top view on a single light entrance facet illuminated by arectangular light pattern;

FIG. 8 is a sequence of top views on the light entrance facets shown inFIG. 7, illuminated with different light patterns at the beginning of ascan process;

FIG. 9 is a sequence of top views of the light entrance facets shown inFIG. 7, illuminated with different light patterns at the end of a scanprocess;

FIG. 10 is a top view on three adjacent light entrance facetsilluminated with different light patterns;

FIGS. 11 a to 11 d are schematic illustrations showing the intensityprofile of the illuminated field along the scan direction;

FIGS. 12 a to 12 c are top views on the optical raster element in whichdifferent numbers of light facets are illuminated with different lightpatterns;

FIG. 13 is a partially perspective view of the mirror array, thecondenser and first and second microlenses of the optical rasterelement, wherein a diaphragm starts moving into the path of light;

FIG. 14 is a flow diagram of a method of operating an illuminationsystem of a microlithographic projection exposure apparatus according tothe disclosure.

DESCRIPTION OF PREFERRED EMBODIMENTS I General Structure of ProjectionExposure Apparatus

FIG. 1 is a perspective and highly simplified view of a projectionexposure apparatus 10 that includes an illumination system 12 forproducing a projection light beam. The projection light beam illuminatesa field 14 on a mask 16 containing minute structures 18. In thisembodiment the illuminated field 14 has approximately the shape of aring segment. However, other, for example rectangular, shapes of theilluminated field 14 are contemplated as well.

A projection objective 20 images the structures 18 within theilluminated field 14 onto a light sensitive layer 22, for example aphotoresist, which is deposited on a substrate 24. The substrate 24,which may be formed by a silicon wafer, is arranged on a wafer stage(not shown) such that a top surface of the light sensitive layer 22 isprecisely located in an image plane of the projection objective 20. Themask 16 is positioned via a mask stage (not shown) in an object plane ofthe projection objective 20. Since the latter has a magnification ofless than 1, a minified image 14′ of the structures 18 within theilluminated field 14 is projected onto the light sensitive layer 22.

During the projection, the mask 16 and the substrate 24 move along ascan direction which coincides with the Y direction. Thus theilluminated field 14 scans over the mask 16 so that structured areaslarger than the illuminated field 14 can be continuously projected. Sucha type of projection exposure apparatus is often referred to as“step-and-scan apparatus” or simply a “scanner”. The ratio between thevelocities of the mask 16 and the substrate 24 is equal to themagnification of the projection objective 20. If the projectionobjective 20 inverts the image, the mask 16 and the substrate 24 move inopposite directions, as this is indicated in FIG. 1 by arrows A1 and A2.However, the present disclosure may also be used in stepper tools inwhich the mask 16 and the substrate 24 do not move during projection ofthe mask.

In the embodiment shown, the illuminated field 14 is not centered withrespect to an optical axis 26 of the projection objective 20. Such anoff-axis illuminated field 14 may be desirable with certain types ofprojection objectives 20. In other embodiments, the illuminated field 14is centered with respect to the optical axis 26.

II General Structure of Illumination System

FIG. 2 is a more detailed meridional section through the illuminationsystem 12 shown in FIG. 1. For the sake of clarity, the illustration ofFIG. 2 is considerably simplified and not to scale. This particularlyimplies that different optical units are represented by very few opticalelements only. In reality, these units may include significantly morelenses and other optical elements.

The illumination system 12 includes a housing 28 and a light source thatis, in the embodiment shown, realized as an excimer laser 30. Theexcimer laser 30 emits projection light that has a wavelength of about193 nm. Other types of light sources and other wavelengths, for example248 nm or 157 nm, are also contemplated.

In the embodiment shown, the projection light emitted by the excimerlaser 30 enters a beam expansion unit 32 in which the light beam isexpanded without altering the geometrical optical flux. The beamexpansion unit 32 may include several lenses as shown in FIG. 2, or maybe realized as a mirror arrangement, for example. The projection lightemerges from the beam expansion unit 32 as a substantially collimatedbeam 34. In other embodiments, this beam may have a significantdivergence. The collimated beam 34 impinges on a plane folding mirror 36provided for reducing the overall dimensions of the illumination system12.

After reflection from the folding mirror 36, the beam 34 impinges on anarray 38 of microlenses 40. A mirror array 46 is arranged in or in thevicinity to a back focal plane 91 of the microlenses 40. As will beexplained in more detail below, the mirror array 46 includes a pluralityof small individual mirror elements M_(ij) that can be tilted,independently from each other, by two tilt axes that are preferablyaligned perpendicularly to each other. The total number of mirrorelements M_(ij) may exceed 100 or even several 1000. The reflectingsurfaces of the mirror elements M_(ij) may be plane, but could also becurved, if an additional reflective power is desired. Apart from that,the mirror surfaces could be provided with diffractive structures. Inthis embodiment the number of mirror elements M_(ij) is equal to thenumber of microlenses 40 contained in the microlens array 38. Thus eachmicrolens 40 directs a converging light bundle on one mirror elementM_(ij) of the mirror array 46.

The tilting movements of the individual mirror elements M_(ij) arecontrolled by a mirror control unit 50 which is connected to an overallsystem control 52 of the illumination system 12. Actuators that are usedto set the desired tilt angles of the mirror elements M_(ij) receivecontrol signals from the mirror control unit 50 such that eachindividual mirror element M_(ij) is capable of reflecting an impinginglight ray by a reflection angle that is variable in response to thecontrol signal. In the embodiment shown there is a continuous range oftilt angles at which the individual mirror elements M_(ij) can bearranged. In other embodiments, the actuators are configured such thatonly a limited number of discrete tilt angles can be set.

FIG. 3 is a perspective view of the mirror array 46 including, for thesake of simplicity, only 8·8=64 mirror elements M_(ij). Light bundles 54a impinging on the mirror array 46 are reflected to different directionsdepending on the tilt angles of the mirror elements M_(ij). In thisschematic representation it is assumed that a particular mirror elementM₃₅ is tilted about two tilt axes 56 x, 56 y relative to another mirrorelement M₇₇ so that the light bundles 54 b, 54 b′ which are reflected bythe mirror elements M₃₅ and M₇₇, respectively, are reflected intodifferent directions.

The mirror array 46 may be replaced by any other deflective structurethat makes it possible to direct light rays impinging on the structureinto various directions, wherein the directions can be changedindividually for different portions of the structure upon application ofa suitable control signal. Such alternative structures may include, forexample, electro-optical or acousto-optical elements. In such elementsthe refractive index may be varied by exposing a suitable material toultrasonic waves or electric fields, respectively. These effects can beexploited to produce index gratings that direct impinging light intovarious directions.

Referring again to FIG. 2, the light bundles reflected from the mirrorelement M_(ij) impinge on a first condenser 58 which ensures that theslightly diverging light bundles impinge, now as at least substantiallyparallel light bundles, on an optical integrator 72 which produces aplurality of secondary light sources. The optical integrator 72increases the range of angles formed between the light rays and anoptical axis OA of the illumination system 12. In other embodiments, thefirst condenser 58 is dispensed with so that the light bundles impingingon the optical integrator 72 have a larger divergence.

The optical integrator 72 is realized, in the embodiment shown, as afly's eye lens including two substrates 74, 76 that each includes twoorthogonal arrays of parallel cylindrical microlenses. Otherconfigurations of the optical integrator are envisaged as well, forexample integrators including an array of microlenses that haverotationally symmetrical surfaces, but rectangular boundaries. Referenceis made to WO 2005/078522 A, US 2004/0036977 A1 and US 2005/0018294 A1,in which various types of optical integrators suitable for theillumination system 12 are described. The function of the opticalintegrator 72 will be explained in more detail further below withreference to FIGS. 5 a and 5 b.

Reference numeral 70 denotes a system pupil surface of the illuminationsystem 12 that substantially defines the angular distribution of thelight impinging on the mask 14. The system pupil surface 70 is usuallyplane or slightly curved and is arranged in or in immediate vicinity tothe optical integrator 72. As the angular light distribution in thesystem pupil surface 70 directly translates into an intensitydistribution in a subsequent field plane, the optical integrator 72substantially determines the basic geometry of the illuminated field 14on the mask 16. Since the optical integrator 72 increases the range ofangles considerably more in the X direction than in the scan directionY, the illuminated field 14 has larger dimensions along the X directionthan along the scan direction Y.

The projection light emerging from the secondary light sources producedby the optical integrator 72 enters a second condenser 78 that isrepresented in FIG. 2 by a single lens only for the sake of simplicity.The second condenser 78 ensures a Fourier relationship between thesystem pupil surface 70 and a subsequent intermediate field plane 80 inwhich a field stop 82 is arranged. The second condenser 78 superimposesthe light bundles, which are produced by the secondary light sources, inthe intermediate field plane 80, thereby achieving a very homogenousillumination of the intermediate field plane 80. The field stop 82 mayinclude a plurality of movable blades and ensures sharp edges of theilluminated field 14 on the mask 16. Blades are not only moved when anew mask having different dimensions shall be projected, but also at thebeginning and the end of each scan process in order to ensure that eachpoint on the mask 16 receives the same amount of light energy.

A field stop objective 84 provides optical conjugation between theintermediate field plane 80 and the mask plane 86 in which the mask 16is arranged. The field stop 82 is thus sharply imaged by the field stopobjective 84 onto the mask 16. As will be explained further below, thefield stop 82 and the field stop objective 84 may be dispensed with inother embodiments.

III Function and Control of the Illumination System

FIG. 4 is a cut-out of FIG. 2 showing the mirror array 46, the firstcondenser 58 and first microlenses 88 formed on the first substrate 74of the optical integrator 72. In this embodiment the microlenses 88 arerotationally symmetrical but have square borderlines. In otherembodiments each microlens is formed by crossing two cylindricalmicrolenses.

As is illustrated in FIG. 4, each mirror element M_(ij) produces a lightbundle L_(ij) which illuminates a small spot 90 on a light entrancefacet 92 of one of the first microlenses 88. The position of the spotsmay be varied by tilting the mirror elements M_(ij). The geometry of thespots 90 depends on, among other things, the optical properties of themicrolenses 40 of the array 38 and the optical properties of the mirrorelements M_(ij). In some embodiments the geometry of the spots 90 iscircular; in other embodiments described further below the geometry isapproximately rectangular and in particular square.

As can be seen in FIG. 4, the diameter D of the spots 90 is smaller thanthe diameter of the entrance facet 92 of the illuminated first microlens88. Generally the total area of each spot 90 illuminated on a lightentrance facet 92 of a first microlens 88 should be considerably, forexample at least 5 times, preferably at least 10 times, more preferablyat least 20 times, smaller than the area of the respective lightentrance facet 92. If the light entrance facets 92 have different areasand each spot 90 can be produced on any of these facets, the maximumarea of the light entrance facet 92 may be taken as reference. If thespots 90 are sufficiently small in comparison to the light entrancefacets 92 of the first microlenses 88, it is possible to producedifferent light patterns on the light entrance facets 92. The lightpatterns may be easily varied by suitably controlling the mirrorelements M_(ij) with the help of the mirror control unit 50.

The effect produced by illuminating different light patterns on thelight entrance facets 92 is elucidated with reference to FIG. 5 a. Thisfigure is an enlarged and not to scale cut-out of FIG. 2 showing theoptical integrator 72, the second condenser 78 and the intermediatefield plane 80. From the optical integrator 72 only two pairs of a firstmicrolens 88 and a second microlens 94 are illustrated for the sake ofsimplicity. Again, the microlenses 88, 94, which are sometimes alsoreferred to as field and pupil honeycomb lenses, may be configured asindividual microlenses having rotationally symmetrical refractivesurfaces and a rectangular borderline, or as crossed cylindricalmicrolenses as shown in FIG. 2, for example. Microlenses 88, 94 have atleast along one direction perpendicular to an optical axis OA of theillumination system 12 a none-zero optical power.

Each pair of adjacent microlenses 88, 94 produces a secondary lightsource. In the upper half of FIG. 5 a it is assumed that converginglight bundles L_(1a), L_(2a) and L_(3a) illustrated with solid, dottedand broken lines, respectively, impinge on different points of the lightentrance facet 92 of the first microlens 88. After having passed the twomicrolenses 88, 94 and the condenser 78, each light bundle L_(1a),L_(2a) and L_(3a) converges to a focal point F₁, F₂ and F₃,respectively. From the upper half of FIG. 5 a it thus becomes clear thatthere is a one to one correspondence between the position where a lightray impinges on the light entrance facet 92 on the one hand and theposition where this light ray passes the intermediate field plane 80 (orany other conjugated field plane). As a result, the dimension of thefield illuminated in the intermediate field plane 80 (and thus the field14 illuminated in the mask plane 86) can be varied by changing theregion illuminated on the light entrance facet of the first microlens88. This region can be changed very effectively with the help of themirror device 46, as has been explained above with reference to FIG. 4.

As a matter of course, these considerations apply separately for the Xand the Y direction. Thus the geometry of the illuminated field 14 canbe varied independently for the X and Y direction by varying theillumination of the light entrance facets 92 separately for the X and Ydirection, respectively. In other words, almost any arbitrary geometryof the illuminated field in the intermediate field plane 80 can beachieved if the area illuminated on the light entrance facet 92 of thefirst microlens 88 is suitably determined.

The lower half of FIG. 5 a illustrates the case where collimated lightbundles L_(1b), L_(2b) and L_(3b) impinge on different regions of thelight entrance facet 92 of the first microlens 88. The light bundles arefocused in a common focal point F located in the second microlens 94 andthen pass, now collimated again, the intermediate field plane 80. Againit can be seen that the region where a light bundle L_(1b), L_(2b) andL_(3b) impinges on the light entrance facet 92 translates into a regionwhich is illuminated in the intermediate field plane.

In FIG. 5 a it is assumed that the system pupil surface 70 is positionedimmediately behind the second microlenses 94. In the case of thestrongly converging light bundles L_(1a), L_(2a) and L_(3a) asillustrated in the upper half of FIG. 5 a, the light bundles L_(1a),L_(2a) and L_(3a) intersect the system pupil plane 70 in a region whichis slightly larger than an exit facet of the second microlens 94. In thecase of collimated light bundles L_(1a), L_(2a) and L_(3a) asillustrated in the lower half of FIG. 5 a, the light bundles L_(1b),L_(2b) and L_(3b) intersect the system pupil plane 70 in a region whichis much smaller than an exit facet of the second microlens 94. In manycases the light impinging on the optical integrator 72 is slightlydiverging, which corresponds to illumination conditions on the lightentrance facets 92 that are somewhere in between what is shown in theupper and the lower half of FIG. 5 a. In such cases the secondary lightsources may have a geometry as shown in FIG. 5 b which is a top view onthe system pupil plane 70. The secondary light sources are indicated bysquares 95; the circle 97 indicates the clear diameter of the systempupil surface. The light emitted from each secondary light source 95will usually have different divergences along the X and the Y directionin order to obtain an illuminated field with an aspect ratio distinctfrom 1.

In the foregoing it has been assumed that all first microlenses 88 areilluminated in the same manner so that the fields illuminated bysecondary light sources 95 superimpose. If the intensity in theilluminated field shall not be constant but shall have a certain profilealong at least one direction, it is also possible to illuminate thelight entrance facets 92 of the first microlenses 88 differently.

Depending on the geometry of the spots 90, it may not be possible toilluminate a continuous region with constant irradiance on the lightentrance facet 92 of the first microlenses 88. For example, if the spotshave the geometry of a circle, the spots may be arranged on the lightentrance facet 92 such that either (small) gaps remain between adjacentspots 90, or such that the spots 90 partially overlap. For that reasonthe region illuminated on the light entrance facets will be referred toin the following as light pattern which may or may not contain gaps andin which different none-zero intensities may occur if two or more spots90 completely or partially overlap.

In one embodiment the spots 90 have a square or rectangular geometrysuch that rectangular light patterns can be produced that have (at leastapproximately) only one none-zero intensity and no gaps between adjacentspots.

This is illustrated in FIG. 6 which is a top view on the opticalintegrator 72 showing an array of 7·7 first microlenses 88. Theborderlines of the first microlenses 88 form a regular grid of squareswhich each can be individually illuminated with different or identicallight patterns. In the configuration shown in FIG. 6 it is assumed thatonly the light entrance facets 92 located within two poles P1, P2, whichare surrounded by thick solid lines, are illuminated. The poles P1, P2are arranged symmetrically on opposite sides of the optical integrator72 and are approximately T-shaped.

In contrast to prior art solutions the poles P1, P2 are not completelyilluminated, but with an rectangular light pattern LP which is repeatedidentically on the light entrance facet 92 of each first microlens 88.Each light pattern 94 is, in turn, assembled from square spots 90 whichare arranged one behind the other along a line. The aspect ratio of eachlight pattern 94 is thus 4:1, with the longer side extending along the Xdirection.

As has been explained above with reference to FIG. 5 a, this will resultin a field illuminated in the intermediate field plane 80 which has alsoan aspect ratio of 4:1 (assuming that there are not anamorphotic opticalelements in between). If the scan direction of the projection exposureapparatus 10 coincides with the Y direction, an illumination of theoptical integrator 72 as shown in FIG. 6 will result in an illuminatedfield 14 which has its short sides extending along the scan direction Yand the long sides extending along the X direction. Since only lightentrance facets 92 within the poles P1, P2 are illuminated, the lightwill impinge on the intermediate field plane 80 exclusively obliquelyfrom opposite sides, as it is characteristically for a dipoleillumination setting.

If the illuminated field shall be half as wide with an aspect ratio of2:1, for example, the left and right spots 90 in each light pattern LPcould be shifted towards the centre so that the two middle positions areilluminated by two spots 90 simultaneously. The illuminated field in theintermediate field plane 80 (and in any subsequent field plane) willthen have an aspect ratio of 2:1 as well, but with twice the radiantpower, because no light is blocked or lost otherwise in the illuminationsystem 12. If the radiant power shall not be doubled, the left and rightspots 90 are simply switched off (e.g. moved away from the opticalintegrator 72). It has to be noted that a reduction of the field size bychanging the light pattern LP does not substantially change the angulardistribution of light impinging on the mask 16.

IV Taking Over Field Stop Function

In illumination systems 12 of projection exposure apparatus 10 of thescanner type the field stop 82 does not only ensure sharp edges (atleast along the long sides of the illuminated field extending parallelto the scan direction Y), but it also increases and decreases the lengthof the illuminated field 14 along the scan direction Y at the beginningand the end of the scan process, respectively. This opening and shuttingfunction is desirable to ensure that all points on the mask receive thesame amount of light energy. To this end the field stop 82 is usuallyprovided with blades that can be moved along the scan direction Y.

In the illumination system 12 the geometry of the field can be varied bychanging the light patterns illuminated on the light entrance facets 92of the optical integrator 72. Thus there is, at least in principle, noneed for such an adjustable field stop 82. Without the field stop 82also the field stop objective 84 may be dispensed with, which results ina very considerable simplification of the overall design of theillumination system 12. It is also possible to have a field stopobjective 84, but only a simplified field stop 82, for example a stopthat delimits only the long edges of the illuminated field 14, while theperpendicular edges are solely determined by the illumination of theoptical integrator 72.

If the field stop 82 and the field stop objective 84 shall be dispensedwith, the opening and shutting function has to be taken over by theremaining components of the illumination system 12, in particular by themirror array 46 and its control 50.

1. First Approach

One approach of achieving this will be explained in the following withreference to FIGS. 7 to 9 which show different light patterns on a lightentrance facet 92 of a single first microlens 88. FIG. 7 shows the lightpattern LP at an instant in the middle of the scan process in which theilluminated field 14 has its maximum extension along the scan directionY. The light pattern LP has the geometry of a rectangle with an aspectratio that corresponds, as has been mentioned above, to the aspect ratioof the illuminated field 14 on the mask 16. The rectangular lightpattern LP is assembled from n spot rows R₁ to R_(n), wherein each rowis assembled from a plurality of spots 90 which are arranged one behindanother along a line parallel to the X direction.

At the beginning of a scan process only the line R₁ is illuminated for atime interval T during which N light pulses are produced by the lightsource 30. This is shown in the left illustration of FIG. 8.

Then the second row R₂ is additionally illuminated for a time interval Tuntil another N light pulses have impinged on the mask 16, see themiddle illustration of FIG. 8. This situation corresponds to a situationin conventional illumination systems in which the field stop 82commences opening at the beginning of the scan process. The process ofconsecutively switching on more and more rows R_(i) is kept on until alln rows R₁ to R_(n) of the light pattern LP shown in FIG. 7 areilluminated, as is shown in the right illustration of FIG. 8. Thiscorresponds to the situation in conventional illumination systems inwhich the field stop 82 is completely open.

The scan process is then continued without amending the light pattern.Then the aforementioned process is reversed, i.e. the rows R₁ to R_(n)are switched off one after another, beginning with the first row R₁until only the last row R_(n) is illuminated, see right illustration ofFIG. 9. If the last row R_(n) is switched off, the scan process isterminated.

The light entrance facets 92 of the remaining microlenses 88 may beilluminated just in the same manner as has been explained above withreference to FIGS. 7 to 9. Since each of the rows R₁ to R_(n) receivesthe same number N of light pulses when the rows are switched on or off,it is ensured that each point on the mask 16 receives exactly the samenumber of light pulses during the whole scan process. The light source30 should thus be synchronized with the mirror control 50 so as to makesure that the conditions with regard to the number of light pulsesreliably prevail.

The switching on or off of the rows R₁ to R_(n) can be easily achievedby setting deflection angles for the corresponding mirror elementsM_(ij) such that the spots to be switched on or off are either directedto the desired location, or are directed to a light absorbing surfaceoutside the optical integrator 72, respectively. If a spot 90 has to beswitched on or off, this involves moving the spot over the lightentrance facet of the optical integrator 72. In order to avoid undesiredperturbations, the spots 90 should be moved during the interval betweensuccessive light pulses. This involves very quick movements of themirror elements M_(ij).

2. Second Approach

Such quick movements of spots 90 over larger distances can be avoided ifthe spots 90 are not completely switched on or off, but are continuouslymoved over the light entrance facet 92 of a single first microlens 88.

This approach will be explained in the following with reference to FIGS.10 to 12:

FIG. 10 is a top view on light entrance facets 92 a, 92 b, 92 c of threeadjacent first microlenses 88. Pairs of blinds 96 a, 96 b, 96 c arefixedly attached to the light entrance facets 92 a, 92 b and 92 c,respectively, such that a central stripe of different height along thescan direction Y remains uncovered. In this embodiment the pairs ofblinds 96 a, 96 b, 96 c are dimensioned such that the uncovered stripein the middle is not restricted along the X direction, but only alongthe Y direction.

For the sake of simplicity it is assumed that one third of the lightentrance facets 92 of the optical integrator 72 is provided with blinds96 a, one third with a blinds 96 a and the remaining third with blinds96 c.

FIG. 11 a shows schematically the intensity distribution that isobtained in the intermediate field plane 80 if equal numbers of thethree types 92 a, 92 b, 92 c of light entrance facets are illuminated.The intensity profile along the Y direction increases until it reaches atop level and then decreases again. This approximates a trapezoidalintensity profile along the scan direction Y. If the number of differentblinds 96 a, 96 b, 96 c is increased, this approximation improves. Theheight and slope of the trapezoidal intensity profile are mainlydetermined by the distribution of blinds 96 a, 96 b, 96 c.

At the beginning and the end of the scan process the intensity profilealong the scan direction Y has to be modified. FIG. 11 b, 11 c and 11 dillustrate how the intensity profile shown in FIG. 11 a is graduallytrimmed from the right side. The trimming process commences with movingthe spots 90 along the scan direction Y on the light entrance facets 92c which are covered by a blind 96 c. The row of spots 90 originallyadjacent one of the blinds 96 c will then be completely blocked by thisblind, which is indicated in FIG. 11 b by a white square 90′. Thisprocess is repeated, but this time also the spots 90 illuminated onlight entrance facets 92 b provided with blinds 96 b are moved along thesame direction. As a result, also a row of spots 90″ is blocked by oneof the blinds 96 b. This situation is illustrated in FIG. 11 c.

If the top level of the intensity profile is reached, also spots 90 onthe light entrance facets 92 a provided with blinds 96 a will be movedalong the scan direction Y. As a result, also a row of spots 90′″ isblocked by one of the blinds 96 a. This situation is illustrated in FIG.11 d. This process has to be repeated in reverse order from the oppositeside at the beginning of a scan process.

The light entrance facets 92 a, 92 b, 92 c covered with the differentblinds 96 a, 96 b and 96 c, respectively, should be distributed over theentrance surface of the optical integrator 72 such that adverse effectson the intensity distribution in the pupil surface 70 are minimized.Without such an optimized distribution it may happen that theillumination angle distribution, which corresponds to the lightintensity distribution in the pupil surface 70, varies when spots 90 aremoved along the scan direction Y during the beginning and end of thescan process.

FIGS. 12 a to 12 c illustrate this principle for a dipole illuminationsetting in which two poles P1, P2 are illuminated on the light entrancefacet of the optical integrator 72. At the beginning and the end of thescan process only the light entrance facets 92 c provided with blinds 96c contribute to the illumination of the mask 16. If the scan processcontinues, also the light entrance facets 92 b provided with blinds 96 bcontribute to the illumination of the mask 16, see FIG. 12 b. Finallyalso the light entrance facets 92 a provided with blinds 96 a contributeto the illumination of the mask 16, see FIG. 12 c.

As can be seen in FIGS. 12 a to 12 c, the different types of lightentrance facets 92 a, 92 b, 92 c are distributed over the poles P1, P2such that the switching on and off of the different types of the lightentrance facets 92 a, 92 b, 92 c does not significantly affect theintensity balance within each pole P1, P2. Such a balance could not beachieved if the different types of light entrance facets were arrangedalong three lines extending from the top to the bottom in FIGS. 12 a to12 c, respectively.

This approach may be combined with the first approach if desired. Then atrapezoidal or any other non-rectangular intensity profile of theilluminated field 14 along the scan direction Y may be achieved, butwithout the blinds 96 a, 96 b, 96 c. A row of spots 90, which shall beswitch off, then does not move into a blind, but is switched off bymoving the spots to an absorbing surface outside the optical integrator72. It has only to be ensured that the time interval between twosuccessive light pulses suffices to move the spots 90 to the absorbingsurface.

3. Third Approach

It is also possible to dispense only with the field stop objective 84whilst using a movable stop blade at another location in theillumination system 12. This is schematically illustrated in FIG. 13which is a cut-out of FIG. 2 showing the mirror array 46, the firstcondenser 58 and the optical integrator 72 with first microlenses 88 andsecond microlenses 94.

The solid lines indicate light rays LM₁ that emerge from micro-mirrorsM_(ij) of a first row RM₁ of micro-mirrors M_(ij). The micro-mirrorsM_(ij) of this first row RM₁ are controlled such that they illuminateexclusively an upper portion of the light entrance facets 92 of thefirst microlenses 88 of the optical integrator 72. As a result, thelight rays LM₁ illuminate a portion of the field 14 on the mask 16 whichis closest to one of its edges.

The broken lines indicate light rays LM₅ which emerge from micro-mirrorsM_(ij) in a middle row RM₅ of the mirror array 46. The micro-mirrorsM_(ij) of this middle row RM₅ are controlled such that the light raysLM₅ illuminate only a middle portion of the light entrance facets 92 ofthe first microlenses 88. As a result, the light rays LM₅ illuminate aline crossing the centre of the field 14 illuminated on the mask 16.

The dotted lines indicate light rays LM₉ which emerge from micro-mirrorsM_(ij) of a last row RM₉ of micro-mirrors M_(ij). The micro-mirrorsM_(ij) of this last row RM₉ are controlled such that the light rays LM₉illuminate only a lower portion of the light entrance facets 92 of thefirst microlenses 88. As a result, the light rays LM₉ illuminate aportion of the field 14 illuminated on the mask 16 which is closest toan edge arranged opposite the edge illuminated by the light rays LM₁.

An opaque blade 98 is arranged in immediate vicinity to the micro-mirrorarray 46 such that it can be moved, with the help of actuators 100,during a scan process along the scan direction Y. If the blade 98 startsmoving downwards as indicated by arrows in FIG. 13, it will firstobstruct the light rays LM₁ emerging from the micro-mirrors M_(ij) ofthe first row RM₁ of micro-mirrors M_(ij). Consequently, the field 14illuminated on the mask 16 will be trimmed from the Y direction. Theblade 98 will then block light rays emerging from the neighboring rowRM₂ of micro-mirrors M_(ij), which results in a further trimming of theilluminated field 14, and so on.

When the blade 98 keeps on moving until it reaches the middle row RM₅,the field 14 illuminated on the mask 16 will have only half of itsoriginal length along the scan direction Y.

It is thus possible, by appropriately selecting the mirror elementM_(ij) that illuminate certain portions of the light entrance facets 92of the first microlenses 88, to vary the geometry of the field 14illuminated on the mask 16 by obstructing mirror elements M_(ij) withthe help of an opaque blade or another diaphragm.

V Operating Method

FIG. 14 is a flow diagram illustrating the main steps of a method ofoperating an illumination system of a microlithographic projectionexposure apparatus in accordance with the present disclosure.

In a first step S1 the illumination system 12 of the microlithographicprojection exposure apparatus 10 is provided. The illumination system 12includes the optical raster element 72 having a plurality of lightentrance facets 92.

In a step S2 light patterns, which are assembled from individual spots90, are produced on the light entrance facets 92 of the optical rasterelement 72.

In a step S3 it is determined that the geometry of the field 14 to beilluminated in the mask plane 86 shall change. A reason for doing thismay be the change of the mask 16, or a scan process which makes itdesirable to open and shut the illuminated field 14 at the beginning andthe end of the scan process, respectively.

Then the light patterns on the light entrance facets 92 are varied byrearranging and/or removal and/or adding spots, as previously described.

The above description of the preferred embodiments has been given by wayof example. From the disclosure given, those skilled in the art will notonly understand the present disclosure and its attendant advantages, butwill also find apparent various changes and modifications to thestructures and methods disclosed. The applicant seeks, therefore, tocover all such changes and modifications as fall within the spirit andscope of the disclosure, as defined by the appended claims, andequivalents thereof.

What is claimed is:
 1. An illumination system having a pupil surface,the illumination system comprising: an optical raster element configuredto produce a plurality of secondary light sources located in the pupilsurface, the optical raster element comprising a plurality of lightentrance facets, each light entrance face being associated with one ofthe secondary light sources; and a beam deflecting device comprising anarray of reflective or transparent beam deflecting elements, each beamdeflecting element being configured to illuminate a spot on one of thelight entrance facets at a position that is variable by changing adeflection angle of the beam deflecting element, the spots illuminatedby the beam deflecting elements having a total area that is at least 5times smaller than a maximum total area of any of the light entrancefacets, wherein: during use of the illumination system, points on thelight entrance facets are imaged onto a mask plane; and the illuminationsystem is configured to be used in a microlithographic projectionexposure apparatus.
 2. The illumination system of claim 1, furthercomprising a control unit configured to control the beam deflectionelements so that variable light patterns assembled from the spots can beformed on at least one of the light entrance facets.
 3. The illuminationsystem of claim 2, wherein the control unit is configured to control thebeam deflection elements so that, at a given instant during use of theillumination system, the light patterns have different lengths along thescan direction.
 4. The illumination system of claim 1, wherein the spotshave an at least substantially rectangular geometry.
 5. The illuminationsystem of claim 1, wherein the system is configured so that the lightpatterns produced on all illuminated light entrance facets can beidentical.
 6. The illumination system of claim 1, further comprising adiaphragm and an actuator, wherein the diaphragm is in close proximityto the beam deflection device, and the actuator is configured to movethe diaphragm parallel to a scan direction.
 7. An apparatus, comprising:an illumination system according to claim 1; and a projection objective,wherein the apparatus is a microlithographic projection exposureapparatus.